Referring to FIG. 11, a description of power supply circuits for use in portable telephones and the like will be given.
Using a voltage of 4V, for example, of battery 101 as the input voltage, it is possible to obtain an output voltage of 2V. Here, coil 102 is called a choke coil. By putting coil 102 in the circuit, a stable output voltage can be obtained. Also, in order to more stabilize the output voltage, it is necessary to increase the inductance of coil 102. In this way, the power supply circuit of FIG. 11 is capable of supplying a DC output voltage which is more stabilized.
Generally, in order to increase the inductance of coil 102, it is necessary to increase the cross-sectional area of the core of coil 102 and to increase the number of turns. This presents a problem of a need to increase the volume of coil 102. On the other hand, in association with the requirement in recent years for a smaller size and lower profile design of portable telephones, there is an increasingly stronger requirement for smaller size and lower profile design of coils for the power supply circuit of portable telephones. For example, coil 102 with an area smaller than 5 mm×5 mm and a thickness of less than 1 mm is being required. Furthermore, the switching frequency has increased from several hundred kHz to several tens of MHz. In association with such a trend toward higher frequencies of the switching frequency, reduction in the core loss is being required. Also, as devices have come to be used at lower voltages and higher currents, there is a case in which a maximum current greater than 0.1 A flows in a coil having a small size and a low profile. For this reason, it is necessary to reduce the resistance of the coil to a lower value.
Japanese Laid-Open Patent Application No. H09-223636 (page 3, FIG. 1) discloses a method for solving these issues.
Referring to FIG. 12, a description of a conventional inductive component will be given. Multilayer magnetic films 112 support coil 111 in a manner sandwiching with the intervention of interlayer insulating layer 115. And through-hole sections (hereinafter “THP”) 114 are provided on the sides and in the center of coil 111. Furthermore, THP 114 is filled with magnetic material 113. Also, as coil 111 is formed by winding a strip of high electric-conductivity materials such as copper, coil 111 can be made thin. However, the above-mentioned coil with a conventional configuration suffered a problem that the inductance could not be increased to a high enough value. Furthermore, as magnetic layer 113 is formed inside THP 114, the cross-sectional area of magnetic layer 113 becomes large. When a current is fed through coil 111, a magnetic flux that vertically penetrates THP 114 is generated, and an eddy current is generated in the horizontal plane of magnetic layer 113. As the cross-sectional area of magnetic layer 113 is large, the eddy current is large.
As a result, the magnetic flux that vertically penetrates THP 114 is reduced.
Consequently, the inductance of the coil cannot be increased. On the other hand, by using a magnetic material having a higher specific resistance, the eddy current can be reduced to a certain extent. However, when the switching frequency increases from several hundred kHz to several tens of MHz, a satisfactory effect of eddy current reduction cannot be obtained. Also, when the diameter of a through hole is 1 mm or smaller, and the depth is 0.1 mm or greater and 1 mm or smaller, for example, it is difficult to fill or dispose a magnetic material into the THP by sputtering or vapor deposition because of difficulties in quality and productivity. The present invention addresses these issues and provides inductive components with which sufficient inductance is obtainable even when designed with a smaller size and a lower profile, and electronic devices that use those inductive components.